(1) Field of the Invention
The present invention relates to an image-forming process for obtaining prints having a grey image. More particularly, the present invention relates to an image-forming process for obtaining a desired grey-scale image from an optional original.
(2) Description of the Related Art
A process for forming prints differing in the image gradation by developing electrostatic latent images by using mixtures of a plurality of toners differing in the color is know.
For, example, Japanese Unexamined Patent Publication No. 52-147444 discloses a process in which an electrostatic latent image is developed with a toner mixture comprising at least two toners of the same polarity but different in the color, in which the difference of the triboelectric charge quantity to a carrier between the toners is 0 to 10 .mu.c/g. In this patent publication, it is taught that not only fundamental color toners but also a black toner, a white toner and a colorless toner can be used as the toners of different colors.
According to this prior art technique, the density or chroma of the color of the reproduced image can be changed by using a mixture of a plurality of toners but it is not clarified how the mixing ratio of the toners and the developing conditions should be set for obtaining a predetermined image density from a certain original. Namely, mutual relations among these factors are not clarified in the above-mentioned patent publication.
In Japan, letters and documents concerning funerals and the like are generally prepared by using a grey ink stick or a grey ink. When such a letter or document is copied according to the conventional copying process, a print having a black image quite different from the grey scale of the original is obtained.